This invention relates to remote monitoring and more particularly to a method and apparatus for establishing the value of a monitored condition at points remote from the monitoring apparatus through the use of two-photon absorption and a range gating technique.
Two photon absorption has been used in the past in spectroscopy expirements to measure the spectra of a gas in a gas cell. This technique, to be described later, measures absorption characteristics of a gas at only one point, at the gas cell. Thus, two-photon absorption has not been available as a tool to measure absorption at remote locations. In fact, the monitoring of atomic or molecular absorption at locations remote from monitoring apparatus has always been difficult and has usually necessitated apparatus at the point of measurement.
However, with the development of range-gated optical radars, the range of an object, even a volume of atoms or molecules, may be determined. Several investigators have used pulses of light and range-gating techniques to stimulate Raman scattering along the path of the pulse projected by an optical radar and to determine the range at which the scattering takes place. This technique involves xe2x80x9csingle photonxe2x80x9d absorption as opposed to xe2x80x9ctwo-photonxe2x80x9d absorption, the significance of which will become clear hereinafter. After xe2x80x9csingle photonxe2x80x9d pumping by the pulse, backscattered radiation is detected along with the time of arrival of the backscattered radiation to obtain an indication of the amount and location of the absorption. The system described has been given the acronym LIDAR (light detection and ranging) and is referred to herein by that name. This work has been exceptionally useful in the area of pollution monitoring due to its range determining capability.
It will, of course, be appreciated that monitoring of the existence or amount of a predetermined pollutant at some distance from the monitoring apparatus provides the basis for detailed pollution concentration maps and therefore provides an accurate basis for pollution control.
While the LIDAR system does, in fact, provide for range-gating, its performance is dependent upon backscattered radiation, a substantial part of which falls off as the inverse square of the distance, (1/r2). Not only is this type system somewhat inefficient and insensitive at large ranges, but also, the measurement of the intensity of the backscattered radiation is highly dependent upon the distance of the pulse from the monitoring apparatus. This is because, as the pulse propagates away from the monitoring apparatus, the measurement must be corrected for the expected 1/r2 decreasing intensity. While the decreased intensity is theoretically calculatable, in practice, it is difficult to get a normalized reading of the backscattered energy over large ranges or distances, even if it is detectable.
Prior to LIDAR, pollution monitors have, in general, measured the total amount of a particular pollutant in a column of gas, in which the column either extends vertically all the way through the earth""s atmosphere or in which the column extends horizontally from one monitoring point to another. In these systems it is impossible to differentiate between pollutant concentrations at any given location in the column, but rather it is an averaging technique which measures the average amount of pollutant over the entire column.
The subject system, instead of utilizing averaging or Raman-scattering, makes use of two-photon absorption which permits range gating without the 1/r2 dependence. In one embodiment, a pulsed laser and a continuous wave (CW) laser are used, both of which produce colliminated beams of energy, with the beam from the pulsed laser being only as long as the pulse. It is the sensing of one of these collimated beams after the two-photon interaction which eliminates the 1/r2 dependence typical of Raman scattering because the collimation is not destroyed by the two-photon interaction. The frequencies of the lasers are set such that the energy of the photons of one laser, hxcexd1, plus the energy of the photons of the other laser, hxcexd2, equals, or nearly equals, that of a transition of the particular pollutant to be monitored.
In order to achieve range gating, two lasers are set up such that the beams from the lasers are xe2x80x9ccounter-propagatingxe2x80x9d in that one beam is propagating in one direction and the other beam is propagating in the opposite direction along the same axis of propagation as the first beam. In the aforementioned embodiment the intensity of the CW beam is continuously read out and correlated with the position of the pulse as the pulse travels along the CW beam, such that the intensity of the CW beam after the two-photon interaction, when correlated with the position of the pulse, specifies the amount of two-photon absorption at the points of overlap or coincidence. The amount of absorption may then be correlated with the amount of pollutant to give pollutant concentration at various known locations. The fact of any absorption at a known location also gives an indication of the existance of a pollutant and its identity.
In general, it is the counter-propagation, the varying points of overlap of energy hxcexd1 and hxcexd2, and the monitoring of one of the collimated beams after the absorption which provides for the relative independence of the measurements on range.
Rather than utilizing pulsed and CW lasers, two continuously operating lasers may be used in which their beams cross in an xe2x80x9cXxe2x80x9d. The point of overlap at the crossing may then be moved in a scissor-like action to vary the point at which the measurement is made. Alternatively, two counter-propagating CW beams may be used in which the frequency of one of the beams is periodically swept such that hxcexd1+hxcexd2 add up to the desired transition of a gas at different points. The different points may be selected by periodically altering the rate of the sweep or by starting a constant rate sweep at different times.
The range-gating is made possible because of the xe2x80x9ctwo-photonxe2x80x9d absorption. It has been found that when a photon of one energy and a photon of another energy add up to the energy associated with a predetermined transition of a gas through which the photons travel, the gas absorbs the energy of both photons regardless of the relative energies of the individual photons, thereby removing these photons from the particular beams. In the subject invention, it will be appreciated that the requisite energies, hxcexd1 and hxcexd2, only exist at the point of overlap of the beams. Thus, absorption only takes place at the point of overlap or coincidence and since this point of overlap is moved along the direction of propagation of one of the beams, it is possible to measure the amount of absorption with position or in one embodiment as against the range of a pulse.
In one embodiment, the pulsed and CW lasers are spaced apart at two stations and pointed at each other through the earth""s atmosphere. In another embodiment, the two lasers, one pulsed and one CW, are located on the same platform. This is called the xe2x80x9csingle-sidedxe2x80x9d embodiment. Spaced from the platform are a number of corner reflecting or retroflective devices which reflect the energy transmitted from the lasers on the platform back along the original path to the platform. In this embodiment the pulsed laser is turned on first to emit a pulse of light and immediately thereafter the CW laser is turned on. The pulse travels the distance from the platform to the corner reflector and is then returned to the platform along the same path. Immediately after the pulse is reflected by the corner reflector it starts to overlap the outgoing CW beam. This establishes counter-propagating beams or more particularly a pulse which counter-propagates with respect to the CW beam. The pulse travels back along the CW beam and at the points of overlap various amounts of absorption occur, with the amount of absorption depending upon the amount of pollutant at the point of overlap. The information as to the amount of absorption is carried on the CW beam which is reflected by the corner reflector and is returned to the platform where an intensity measurement is made. The intensity or other two-photon absorption related characteristic of the CW beam is monitored at the platform and is correlated with the range of the pulse from, in this case, the corner reflector. It will be noted that the beam intensity measurement is made after the two-photon interaction in terms of the number of photons removed from the beam during the interaction.
While the subject invention is useful in monitoring the existance and extent of pollutants in the earth""s atmosphere, it will be appreciated that the subject system may be utilized to localize and detect two-photon absorption associated with any absorbing species occurring at a given position in the earth""s atmosphere. As will be described, two-photon absorption may also result in stimulated emission which is collimated within and carried by one of the beams. The stimulated emission resulting from the two-photon absorption may thus be detected at the monitoring station with the arrival of the xe2x80x9ccarrierxe2x80x9d beam and correlated as to its position.
It is therefore an object of this invention to provide improved remote measurements utilizing two-photon absorption.
It is another object of this invention to provide improved pollution monitoring apparatus for measuring the existance and/or concentration of various pollutants in the earth""s atmosphere at selectable remote locations.
It is a further object of this invention to provide range gated absorption measurements through the use of two-photon absorption and coherent beams which overlap at selectable points along a path.
It is yet another object of this invention to provide a range gating system utilizing two-photon absorption.
These and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which: